1. Field of the Invention
The invention relates to the technical field of image processing and, more particularly, to a bit synchronization method and system for a GPS.
2. Description of Related Art
In a global position system (GPS), a data bit to be transmitted is processed by spread spectrum with twenty coarse/acquisition (CA) codes. A receiver receives a satellite signal, and performs a radio-frequency decimation on the satellite signal to obtain an intermediate-frequency (IF) signal. Then, an analog-to-digital converter (ADC) is used to sample the IF signal, and signal processing is employed to obtain the bits of the corresponding source signal for positioning. Even in a low signal-to-noise ratio (SNR) environment, it is required to complete CA synchronization and bit synchronization, so as to be able to complete data demodulation. The purpose of bit synchronization is to provide a receiver with a clock synchronization signal having the best demodulation capability. The accuracy of the synchronization signal is one of the most important parameters to influence the sensitivity of the receiver.
FIG. 1 shows a typical flow of performing bit synchronization using a bar chart. As shown in FIG. 1, the respective transition numbers of twenty CA-codes are counted in an observation time period to accordingly find a start time of correct bit by setting a threshold. As shown in FIG. 2, in a high SNR environment, the CA-code 2 (CA2) has a counted number higher than the high threshold TH_H, and accordingly it is determined that the bit synchronization occurs at CA2. However, as shown in FIG. 3, in a low SNR environment, the probability of CA transition is close to 50%. Thus, the aforementioned statistical bar chart algorithm is likely to cause a detection error, which further causes a failure of signal demodulation. In addition, the prior art requires the accumulation of the statistical samples, so the observation time may significantly increase to result in prolonging the tracking time, slowing down the response of the entire system, and being unable to position fats.
Therefore, it is desirable to provide an improved bit synchronization method and system to mitigate and/or obviate the aforementioned problems.